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Seismic Acquisition Modeling

Conventional offshore seismic acquisition techniques record data over a useable frequency range of typically 8 to 80 hertz. A seismic section with this bandwidth can resolve reflectors separated by about 20 to 25ms. This resolution is adequate for mapping medium to large geological structures with simple stratigraphy. However, in areas having complex geological features or stratigraphy, conventional data hides the details required to asses the exploration potential.

In many cases it is the seismic acquisition method that has limited the resolution. The source and streamer tow depths are the key acquisition parameters affecting the bandwidth of the recorded data. As the tow depths are decreased, the sea surface ghost reflection notch filter effect is raised to higher frequencies, preserving a wider usable signal bandwidth. This wider bandwidth produces high resolution images, providing the bandwidth is preserved during the recording and processing stages. In suitable areas, a two to three times resolution improvement over conventional data is possible. 

The Seismic Acquisition Modeling system determines the acquisition response of different configurations. Input parameters include gun depth, streamer depth, water depth, sea floor reflectivity, source wavelet type and bandwidth.

                       
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Acquisition Parameters and Seismic Resolution

The source and streamer tow depths are the main acquisition parameters affecting seismic resolution. The source pulse, sampling interval, recording filters and other parameters must also be tailored to the desired acquisition bandwidth.

          
Figure 1.  The Acquisition Parameter Input Form.

The modeled acquisition parameters are specified on the parameter entry form (Figure 1). The filtering effect of the source depth, streamer depth and water layer reverberation can be modeled.

          
Figure 2.  The Amplitude Spectrum of an 11ms Source Pulse.

The amplitude of an 11ms source pulse is shown in figure 2. This pulse contains signal frequencies over 140Hz.

          
Figure 3.  The Source Pulse is Affected by the Ghost Filter, Limiting the Useable Bandwidth.

If the source is towed at a depth of 9m, the phase reversed water surface (ghost) reflection is added to the source pulse with a travel time delay of 12ms. The amplitude spectrum of this modified source pulse is shown in figure 3. The filtering effect of the ghost reflection limits the useable bandwidth to a zone between 0 and 80Hz.

In suitable areas a signal frequency range up to 400Hz or more can be preserved on the processed data to reveal very fine geological detail. The resolution improvement of a factor of three or more is often necessary to identify significant geological features and relationships.

Links:

Click here to download the marine seismic acquisition modeling software. (Seisac11.zip 2137KB)

Click here to download the zipped sections file (Sections.zip 5KB)